Sealed rechargeable batteries have been used for a number of years in applications requiring portable, renewable power. One example of such an application is in the field of portable, hand-held power tools. Another example is in the field of electric vehicles, which typically employ a bank of rechargeable lead-acid batteries. In both cases, the batteries used to power these devices must be recharged on a regular basis.
One problem with recharging batteries, such as sealed lead-acid batteries, is that the useful recharge cycle life of each battery is relatively short. While the present assignee has manufactured sealed lead-acid batteries capable of reaching 300 charging cycles, the typical sealed lead-acid battery can rarely surpass 150 charging cycles. This is in part due to short circuits that form in the battery over time due to manufacturing defects or movement of the electrode plates due to vibration and/or impact damage. Short circuits also result from volumetric changes that occur in the battery plates during discharging and recharging. The present inventors discovered that the battery plates also tend to produce moss-like protuberances after repeated charging cycles, and such plate mossing eventually causes shorting between the electrodes of adjacent cells, between electrodes and cell terminals, or between electrodes of the same cell through the separator material. Plate mossing tends to get progressively worse with every charge cycle.
Another problem with rechargeable batteries is that the charging load applied to the battery is sometimes difficult to regulate. Overcharging is one common problem associated with recharging batteries. In the case of an electric vehicle, for example, where the entire bank of batteries would be recharged simultaneously, it is not uncommon for one of the batteries to have a higher remnant charge than the remaining batteries. When recharging such a bank of batteries, that one battery would tend to be overcharged relative to the remaining batteries. One approach to avoiding overcharging such fully charged batteries is to employ a battery management system (BMS), which diverts the charging current around any batteries in the bank that are fully charged. Such systems, however, are very expensive.
While most batteries are engineered to withstand certain levels of overcharging, heretofore it has been very difficult to prevent the sometimes catastrophic results associated with cases of extreme overcharging. For example, it is possible for the battery case to explode if the generation of gases within the battery plenum exceeds the venting capacity of the battery. Alternatively, the gases generated within the battery during charging could be ignited if a short circuit condition has formed in the battery since the last charging cycle. Such plenum destabilization during charging represents a major safety problem that the battery industry has attempted to solve for many years.
Short circuit situations within the battery also cause excessive heat generation within the battery, particularly in the immediate proximity of the short circuit. Such excessive heat generation can cause degradation of the electrode plates, which, of course, would render the battery inoperative.
As mentioned briefly above, yet another problem with batteries in general is their inherent sensitivity to external vibration. Lead-acid batteries in particular are susceptible to deterioration in performance when subjected to severe vibrations, such as those experienced during shipping and handling, as well as during use. Accidental impacts of the battery can cause shifting of the plates which in turn would cause, at least eventually, short circuit conditions within the battery. Again, such short circuit conditions cause excessive heat and/or sparking during charging, which in turn can result in catastrophic failure of the battery.
It has been known in the art to apply adhesive to the base of the battery plates and battery case to enhance the vibration resistance of the battery. This method, however, fails to address the problem of electrode-to-electrode contact, especially due to plate mossing as first discovered by the inventors.
U.S. Pat. No. 4,777,101 discloses a sealed lead-acid battery that includes plastic partition walls formed in situ. This construction will likely increase the vibration resistance of the battery. However, it will also accentuate the problems that might occur during overcharging, since the gas generated during overcharging cannot escape the monolithic block of plastic within the battery case. Specifically, since the material making up the walls is cured, it cannot crack easily or melt within the battery to allow the gas to escape. Consequently, the threat of battery case explosion is more likely.
It would be desirable to provide a rechargeable battery having improved recharging cycle life, improved safety during overcharging, increased shock and vibration resistance, improved thermal stability, and improved control and containment of generated gases within individual cells. These features become especially important for consumer products applications, because the end user undoubtedly would like to minimize the need for battery replacement and be assured of the safety of the battery during recharging.